This invention relates generally to color correction techniques used in digital color printing and more particularly to improve the color print quality of color printers when reproducing images that have been scanned by a digital scanner. The present invention provides a method and means for creating a high-resolution reproduction of an image that more closely matches the color and tone of an existing photograph made with a traditional photo-finishing process.
Scanners are devices that transform an optical image of a document into electronic form. Scanners typically have lenses and mirrors that focus an image of the page to be transformed onto a photo sensor array, a lamp to illuminate the page to be transformed, and a photo sensor array that coverts the image into an electronic signal. The photo sensor array is typically a charged-coupled device (CCD). The area of the page focused onto the CCD is usually referred to as the xe2x80x9cscan linexe2x80x9d. The direction along the scan line is typically called the scan width or X-direction. The entire object is scanned by sweeping the illuminated scan line across the entire object, either by moving the object with respect to the illumination and optical assemblies, or by moving the illumination and optical assemblies relative to the object. The direction of relative movement is typically called the scan length or Y-direction.
The native or optical resolution of a scanner in the x direction is typically the size or area of the page that is imaged onto the photo sensor element. For an 5.3-micron wide photo sensor with an 8 to 1 magnification ratio in the optical path, the width on the page is {fraction (1/600)}inch (0.0423mm), giving the scanner a native resolution of 600 samples per inch (spi) (23.622 samples per mm).
The optical or native resolution of a scanner in the y direction is a function of the area of the page that is imaged onto the photo sensor element, the sweep rate of the photo sensor element with respect to the page, and the exposure time of the photo sensor. For example, a scanner that has an image of the photo sensor on the page of {fraction (1/600)}inch, a sweep rate of {fraction (1/600)}inch in 5 milliseconds, and an exposure time of 5 milliseconds, would have a native scan resolution in the y direction of 600 samples per inch.
Today most scanners capture the scan information in red, green, and blue colors. The scan data is 3 numbers representing the amount of red, green, and blue light detected at each sample area on the scan. The numbers vary in range depending on the bits used in representing each color. For example, an 8-bit number can range between zero and 255 in value where a 12-bit number can range between zero and 4096. A 24-bit scan would have three 8-bit color values for a total of 24 bits of information. Each of the three color values in the 24-bit scan could represent 256 levels of that color.
Today a user can digitally reproduce an image of a color photograph in two different ways. The first way is to scan an existing photograph and then print the image using a color printer. This method can match the color of the photograph fairly well but may lack some of the detail that was contained in the original negative used to create the color photograph. This is especially true if the user wants to enlarge the size of the original image.
The second method is to scan the original negative of the color photograph and then print an image on a color printer. With this method the detail of the original negative is captured and can be reproduced. However the color of the reproduced image rarely matches the color of the photograph created using traditional photo-finishing methods. The color miss-match between the reproduced image and the original photograph can be quite large. Because the scanned image is digitized, some tone and color adjustments are possible using image editors, but there is no simple linear transformation that will fix all errors without introducing other errors. This leaves the user with a choice between a low detail, well-matched color reproduction or a high detail, poorly matched color reproduction. There is a need for a system that provides a high detail, well-matched color reproduction of a color photograph.
A method is disclosed that creates a high detail reproduction of an image that closely matches the color and tone of an existing photograph. The image detail is retrieved by scanning the original negative of the image. Scanning an existing photograph of the image retrieves the color and tone information. The image detail information and the color and tone information are then combined to create a high detail reproduction of the image that closely matches the color and tone of the existing photograph.